The 36-year-old man was sleeping when the hole opened beneath him.

The earth opened up and swallowed a Hillsborough County man late Thursday in a terrifyingly rare reminder of the unpredictable dangers lurking just below Florida's surface.

Central Florida — particularly the Interstate 4 corridor from Volusia County to Hillsborough County — is more susceptible to sinkholes than the rest of the state because of its geology.

Our subdivisions, shopping malls, schools and roads are built on Florida's spongelike crust of limestone, which is full of cavities and cracks that sometimes collapse.

"This is a hidden danger," Manoj Chopra, a University of Central Florida civil-engineering professor, said Friday as word of the killer sinkhole spread across the globe.

Sinkholes form when water dissolves limestone, causing sands to migrate through and form a hole on the surface — like the neck of an hourglass.

"It's like someone pulling the plug on a drain and swoosh, everything circles down the hole," said Mike Perkins, curator of the Orange County History Center's sinkhole exhibit. "This is almost like an earthquake; it just happens."

The region has a history of activity, including a 320-foot-wide, 90-foot-deep sinkhole that swallowed a Winter Park car dealership and home in 1981. It is now a pond near Fairbanks Avenue.

Three years ago, sinkholes collapsed lanes of U.S. Highway 27 in east Polk County. And several years before that, a sinkhole that opened up on Scott Lake swallowed enough water to make the shoreline recede dramatically in a Lakeland-area exclusive neighborhood.

Chopra said activity occurs under the surface constantly that we cannot see.

"A large number of these will not show you any type of symptoms on the surface at all," he said.

And though there's no sure way to predict sinkholes, researchers know that heavy rainfall or extreme drought can cause sinkholes to accelerate.

Though they generate attention, sinkholes are relatively infrequent, said Hal Wilkening, director of the division of water resources for the St. Johns River Water Management District.

Areas west of Orlando generally experience more sinkholes than the east side, experts said.

"It's like being struck by lightning," he said. "It's a low-risk occurrence," but it can be devastating.

Experts could not recall the last time they heard about a loss of life related to a sinkhole.

Jeremy Bush said he heard a crash and the screams of his 36-year-old brother, Jeffrey Bush, late Thursday.

The earth opened up beneath Jeffrey Bush's bedroom, swallowing his floor, his dresser and his bed as he was sleeping inside the Tampa-area home.

Running to help, all Jeremy Bush could see was his brother's mattress sticking up out of the center of a large, dark sinkhole threatening to take the entire house into the depths.

"I thought I could hear him hollering for me to help him," Bush told reporters Friday. Jeremy Bush said he jumped into the collapsing hole hoping to save his brother and used a shovel to dig.

But Hillsborough County deputies pulled him out before he was sucked in farther.

Authorities condemned the concrete-block home, determining the ground was unstable. Surrounding homes were evacuated, but the hole is isolated to the house, authorities said.

Hillsborough Fire Rescue officials lowered a camera and listening device into the 20-foot-deep hole to try to find Jeffrey Bush. But the ground kept moving and they lost the equipment.

"He's down there, but we can't hear here anything and we can't see anything," said Ronnie Rivera, a Hillsborough County Fire Rescue spokesman. "We just can't do anything."

Structural engineers brought in equipment to determine if rescuers can enter the house. But with each hour that passed, the hope for rescue faded and despair set in.